Glitches are anomalies in software programs, such as video games. They can cause various problems ranging from the purely graphical and audio glitching to completely corrupting or deleting the save file.

Glitches can be triggered from problems within a game's code, or players doing things the programmers didn't anticipate, therefore causing the game to react unexpectedly.

Glitch Pokémon

A glitch Pokémon is a Pokémon not intended to be part of the game, but can be accessed through the use of glitches. All glitch Pokémon are either filler data, the game attempting to read an empty hexadecimal address, or data that exists from beta testing.

Glitch characteristics

Glitch moves

A glitch move is a move not intended to be part of the game, but can be accessed through the use of glitches. In Generation I, many glitch moves are named after TMs or HMs; TMs numbered 01 to 55 and HMs numbered 01 to 05 exist as moves; however, some have no name or a glitched, unreadable name. Some glitch moves are of known glitch types, but others have either no readable type or an unknown type. Usually only glitch Pokémon will learn glitch moves.

One way of teaching a Pokémon in Generation I a glitch move is with a Pokémon that can evolve by trading. This can be achieved by trading a trade evolution Pokémon from a Generation I game to a Generation II, at a level where its evolved form will learn a move not in Generation I. Trading the Pokémon back to the Generation I game will cause the move to become a glitch move. For example, trading a level 48 Haunter from Pokémon Red to Pokémon Gold will make the Haunter evolve into Gengar. Since it is level 48, it will learn Mean Look. If it is then traded back to Pokémon Red, it will still have the move, but the game won't recognize it properly since it is a Generation II move, so will become TM12.

Glitch types

There are many different glitch types that are found to be the types of several glitch Pokémon and moves. The majority of them are used for very few Pokémon or moves.

Glitch locations

There are several locations which can only be reached by way of a glitch in the games. One of the most well known glitch locations is Glitch City. Other examples of this are areas in the Sevii Islands that are retrievable via their index number pointer, however, do not have any other data. Sevii Isles 8 and 9 are the only index number areas which have actual map data. There are also other beta locations such as the beta Safari Zone.

Glitch items

Placeholder items are often left in the game's code to prevent it from crashing if the data is accessed, such as the Teru-sama item in Generation II. Other generations likewise have placeholder items, with most appearing at the end of the item list, though a few, likely those removed during development, appear in the middle.

Thus far, glitch items that become real items from a later game only appear in Generation II and Generation IV, as all items not present in Pokémon Ruby and Sapphire that were reintroduced in Pokémon FireRed and LeafGreen and newly-introduced in Pokémon Emerald appear at the end of the list. They would cause a copy of Ruby or Sapphire to crash when it is selected, as they are beyond its item list. Generation II is the only generation in which all new items replace glitch items in the middle of the item list, thus not causing a permanent problem if transferred, as the Griseous Orb is the only item in Generation IV that is programmed at the same index number that a glitch item is present at in Pokémon Diamond and Pearl, while the other items introduced in Pokémon Platinum and reintroduced in Pokémon HeartGold and SoulSilver are after the end of the Diamond and Pearl list, and thus crash the game if the glitch item in their place is selected in a game they do not exist in.